One of the features of the fats and oils from fish and aquatic mammals is their high content of long chain highly unsaturated fatty acids such as DHA (docosahexaenoic acid) or EPA (eicosapentaenoic acid) which are not contained in the fats of terrestrial animals or in vegetable oils. Recently, these highly unsaturated fatty acids have been recognized to be highly effective in the prevention or treatment of cardiovascular diseases such as myocardial infarctions and cerebral thrombosis. In addition, it has been reported that these highly unsaturated fatty acids have antineoplastic activity, and are effective in preventing memory and learning disorders, improving visual functions, preventing allergies, treating diabetes, and in rejuvenating skin.
Therefore, fish are being given a second look as health food. Fish oils which are rich in highly unsaturated fatty acids are refined so as to deodorize, decolorize, and further raise the content of highly unsaturated fatty acids. This refined fish oil is being supplied in pure form, or in an admixture with other food stuffs, as a medicinal drug or a dietary supplement.
The most important problem arising from the addition of refined fish oil to everyday foods so that the highly unsaturated fatty acids help maintain health, is the fishy odor. That means fish oil, even a highly refined one, is oxidized while it is stored so as to emit a strong peculiar odor. Accordingly, even though fish oil does not smell at the stage when it is added to foods, it often becomes odorous during storage, and exhibits an extremely deteriorated flavor and taste.
Various attempts have been made in the past to eliminate such a fishy odor. For example, Japanese Patent Publication No. 3-72264 discloses that an emulsified fish oil is fermented, then added to foods, and Japanese Patent Laid-open No. 6-90662 discloses a process in which a fish oil containing DHA or EPA is blended with stock milk or a mix for producing a cultured dairy product to be fermented by lactic acid bacteria or enzyme, or by a process in which fish oil is blended with dairy products which have been fermented by lactic acid bacteria. But in reality, it has been difficult to control the fishy odor completely or prevent the further generation of the fishy odor by only fermenting the emulsified fish oil, or by simply blending the fish oil with dairy products, since the fish oil components change with time.
Although it may be meaningful to add a refined fish oil to yogurt, which is commonly consumed food, since the addition will improve the shelf-life of the yogurt while facilitating the continuous ingestion of highly unsaturated fatty acids, even when refined fish oil is added to yogurt with such goals in mind, the occurrence of fishy odor is unpreventable if the yogurt is stored for a long time. Moreover, the flavor and taste of the yogurt which are originally plain and simple, are considerably spoiled.
Therefore, an object of the present invention is to examine yogurt containing highly unsaturated fatty acids which are very significant from the view point of nutrition science, and yet have not yet been widely used due to the deterioration of flavor and taste by the development of the fishy odor during storage as described above in order to provide a yogurt containing highly unsaturated fatty acids while having good flavor and taste, and excellent stability in product quality (which will not have a fishy odor) during a normal storage period.